There is increasing evidence that regulation of receptor function is abnormal in many neurologic and metabolic diseases. We have discovered that the membrane-bound acetylcholine receptor (AChR) is phosphorylated in situ by a membrane protein kinase and that receptor phosphorylation is stimulated by K ion and inhibited by cholinergic ligands. Such a covalent biochemical modification of the AChR is likely to be of fundamental importance for regulating the function of the acetylcholine receptor at the synapse. The plan in this proposal is to characterize the regulation of AChR phosphorylation and dephosphorylation at the synapse and to examine the relationship between cholinergic ligands and receptor phosphorylation. Using autoradiographic and immunoprecipitation techniques we will study the effect of cholinergic agonists and antagonists on the two enzymes which regulate the level of phosphorylation of the acetylcholine receptor, membrane protein kinase and phosphoprotein phosphatase. Using millipore filtration methods and equilibrium dialysis with radiolabeled cholinergic ligands, we will determine whether phosphorylation changes the binding affinity of the AChR for cholinergic agonists and antagonists. Finally using "excitable" vesicles prepared from Torpedo californica we will study the role of receptor phosphorylation in regulating 22Na ion efflux and mediating carbachol induced desensitization. All of these experiments will help us to understand the regulation of AChR phosphorylation and to determine whether phosphorylation of the AChR regulates ion permeability at the synapse or mediates desensitization of the receptor.